The capacity of the sarcoplasmic reticulum for phospholipid synthesis: a developmental study

The activities of three enzymes involved in phospholipid synthesis, sn-glycerol-3-phosphate acyltransferase (EC 2.3.1.15), cholinephosphate cytidylyltransferase (EC 2.7.7.15), and cholinephosphotransferase (EC 2.7.8.2), were assayed in adult skeletal muscle. The acyltransferase and cholinephosphotransferase were concentrated in the sarcoplasmic reticulum, where their specific activities were 80 and 33%, respectively, of the specific activity in liver microsomes. Cytidylyltransferase activity was distributed throughout the cell with most of the activity in the cytosol. Its activity in muscle was only 10% of liver activity. Functional sarcoplasmic reticulum was isolated by density gradient centrifugation after calcium loading in the presence of phosphate. The specific activities of these enzymes wee undiminished in the calcium-loaded fraction, suggesting that these enzymes are intrinsic components of the sarcoplasmic reticulum. In developing muscle (2 and 6 days postnatal) acyltransferase and cholinephosphotransferase activities were also present in a calcium-loaded microsomal subfraction at the same level as in the adult. Cytidylyltransferase activity, on the other hand, was 8-fold higher in developing muscle. In addition, developing muscle had a 3-fold increase in the proportion of cytidylyltransferase associated with the microsomal fraction. These data suggest that sarcoplasmic reticulum has the capacity for phospholipid synthesis in mature and developing muscle, and that the rate of phosphatidylcholine synthesis may be regulated by the levels of cytidylyltransferase and by translocation of this enzyme between the sarcoplasmic reticulum and the cytosol.